Recently, with practical implementation of a blue light-emitting diode (LED), development of a white LED using this blue LED is being aggressively pursued. The white LED ensures low power consumption and extended life compared with existing white light sources and therefore, its application to liquid crystal panel backlight, indoor or outdoor lighting devices, etc., is expanding.
A white LED developed at present is obtained by applying a Ce-doped YAG (yttrium-aluminum garnet) onto the surface of a blue LED. However, the fluorescence peak wavelength of Ce-doped YAG is in the vicinity of 560 nm and when this fluorescence color and the light of blue LED are mixed to produce white light, the white light is slightly blue-tinted. Thus, this kind of white LED has a problem of bad color rendering.
To cope with this problem, many (oxy)nitride phosphors are being studied and among others, an Eu-activated α-SiAlON phosphor is known to emit fluorescence (from yellow to orange) with a peak wavelength of around 580 nm that is longer than the fluorescence peak wavelength of Ce-doped YAG (see, Patent Document 1). When a white LED is fabricated by using the α-SiAlON phosphor above or by combining it with a Ce-doped YAG phosphor, a white LED giving a bulb color with a lower color temperature than a white LED using only Ce-doped YAG can be produced.
Furthermore, a white LED having good color rendering property and good color reproducibility is demanded, and development of a white LED combining a green phosphor and a red phosphor with a blue LED is being pursued. However, since the light emitted by the existing red phosphor contains a large amount of light of 700 nm or more, there is a problem that the luminous efficiency deteriorates. On this account, a phosphor that emits an orange to red fluorescence having a peak wavelength of approximately from 600 to 630 nm is required as the red phosphor.
With respect to the Ca-containing α-SiAlON phosphor activated with Eu, represented by the formula:CaxEuySi12−(m+n)Al(m+n)OnN16-n,only a phosphor emitting a fluorescence having a peak wavelength of 580 to 605 nm has been developed as a phosphor with high luminance enough for practical use, and a phosphor having a peak wavelength of more than 605 nm and ensuring high luminance sufficient for practical use has not been developed yet.
Patent Document 2 discloses a phosphor exhibiting excellent luminous efficiency and having a fluorescence peak at a wavelength of 595 nm or more, and a production method thereof, where a smooth-surface particle larger than ever before is obtained by adding a previously synthesized α-SiAlON powder as a seed crystal for grain growth to the raw material powder and a powder having a specific particle size is obtained from the synthesized powder without applying a pulverization treatment.
Specifically, an α-SiAlON phosphor which is an α-SiAlON phosphor having a composition of (Ca1.67,Eu0.08) (Si,Al)12(O,N)16 [wherein x+y=1.75, O/N=0.03] and in which the peak wavelength of the fluorescence spectrum obtained when excited with blue light of 455 nm is from 599 to 601 nm and the luminous efficiency (=external quantum efficiency=absorptivity×internal quantum efficiency) is from 61 to 63%, is disclosed.
However, in the document above, specific examples of a phosphor having a florescence peak wavelength of more than 601 nm and exhibiting a practicable luminous efficiency are not shown.
Patent Document 3 discloses: a light-emitting device characterized by using a phosphor containing an α-SiAlON as a main component, represented by the formula: (Caα,Euβ) (Si,Al)12(O,N)16 (provided that 1.5<α+β<2.2, 0<β<0.2 and O/N≦0.04), and having a specific surface area of 0.1 to 0.35 m2/g; a vehicle lighting device using the same; and a headlamp.
The document above discloses working examples of an α-SiAlON phosphor, where the peak wavelengths of the fluorescence spectra obtained when excited with blue light of 455 nm are 592, 598 and 600 nm, and it is reported that the luminous efficiencies (=external quantum efficiency) thereof are 61.0, 62.7, and 63.2%, respectively.
However, in the document above, specific examples of a phosphor having a fluorescence peak wavelength of more than 600 nm and exhibiting a practicable luminous efficiency are not shown.
Patent Document 4 discloses a Ca-containing α-SiAlON phosphor powder represented by the formula: CaxEuySi12−(m+n)Al(m+n)OnN16−n (provided that 1.37≦x≦2.60, 0.16≦y≦0.20, 3.6≦m≦5.50, 0≦n≦0.30, and m=2x+3y), which is obtained by firing a mixture of a silicon nitride powder, a europium source and a calcium source in an inert gas atmosphere to previously obtain a Ca-containing α-SiAlON precursor, mixing an aluminum source with the Ca-containing α-SiAlON precursor, again firing the mixture in an inert gas atmosphere to obtain a fired Ca-containing α-SiAlON, and further heat-treating the fired product in an inert gas atmosphere, and a production method thereof.
The document above discloses working examples of a Ca-containing α-SiAlON phosphor in which the peak wavelength of the fluorescence spectrum obtained when excited with blue light of 450 nm is from 602 to 605 nm, and it is reported that the luminous efficiency (=external quantum efficiency) thereof is 54% or more.
However, in the document above, specific examples of a phosphor having a fluorescence peak wavelength of more than 605 nm and exhibiting a practicable luminous efficiency are not shown.
Patent Document 5 discloses a SiAlON phosphor having a specific property of emitting light with high luminance compared to conventional phosphors, which is obtained by firing a metal compound mixture capable of composing a SiAlON phosphor when fired, in a specific temperature range in a gas at a specific pressure, then pulverizing the fired product to a specific particle size, and thereafter subjecting the powder to classification and a heat treatment, and a production method thereof.
However, the matter specifically disclosed in the document above is only the peak luminous intensity and since the peak luminous intensity varies depending on the measuring apparatus and measurement conditions, it is not known whether a luminous intensity high enough for practice use is obtained.
Patent Document 6 describes a Ca—Eu-α-SiAlON represented by the formula: (Cax,Euy) (Si12−(m+n)Alm+n) (OnN16-n), obtained by partially substituting the Ca site of a Ca-α-SiAlON with Eu2+, and it is stated that when the SiAlON phosphor satisfies a configuration where x, y, m and n are in the range of 0.5≦x<2.0, 0<y<0.4, 0.5<x+y<2.0, 1.0≦m<4.0 and y≦n<(x+y) and when the starting material composition of the Ca-α-SiAlON falls in the range between two composition lines of Si3N4-a(CaO.3AlN)-bEuO and Si3N4-c (Ca3N2.6AlN)-bEuO, and a, b and c are in the range of 0.5≦a<2.5, 0<b<0.4 and 0.15≦c<0.85, a SiAlON phosphor powder having a peak wavelength of 593 to 620 nm is obtained.
However, the document above merely discloses the peak luminous intensity and since the peak luminous intensity varies depending on the measuring apparatus and measurement conditions, it is not known whether a luminous intensity high enough for practice use is obtained.